Cases of Potomac horse fever (PHF) are starting to appear in Ohio, according to Catherine Kohn, VMD, Dipl. ACVIM, professor in the Department of Veterinary Clinical Sciences at The Ohio State University’s College of Veterinary Medicine. The hospital has recently seen three cases and diagnosed another via mail-in sample. One horse was euthanatized because of laminitis.

"We’re definitely starting to see cases right now," Kohn said, "indicating that the organism is around and active in our area."

The epidemiology of Neorickettsia risticii, the causative agent of PHF, is still under investigation. Ingestion by the horse of aquatic insects such as caddisfles, mayflies, damselflies, and dragonflies, which harbor the trematode host of N. risticii, is probably the most important avenue of transmission of this pathogen under natural circumstances, according to Kohn. The trematode can also infest certain species of snails. Measures to prevent infections should include minimizing exposure to aquatic insects by reducing insect populations in barns (turning out lights, screening stalls, and using insecticides. When the disease is active in your area, limiting access of horses to streams, ponds, and irrigated pastures is also advised.

The available vaccines for PHF, while not perfect, may be a useful option for horse owners in endemic areas, Kohn said